Alpha adrenergic receptors are plasma membrane receptors which are located in the peripheral and central nervous systems throughout the body. They are members of a diverse family of structurally related receptors which contain seven putative helical domains and transduce signals by coupling to guanine nucleotide binding proteins (G-proteins).
The alpha adrenergic receptor family of adrenergic receptors (AR) consists of two groups: alpha-1 and alpha-2. Of the alpha-2 group, there are three distinct subtypes denoted alpha-2A, alpha-2B and alpha-2C. The subtypes are derived from different genes, have different structures, unique distributions in the body, and specific pharmacologic properties. (Due to localization of the genes to human chromosomes 10, 2 and 4, the alpha-2A, alpha-2B, and alpha-2C receptors have sometimes been referred to as alpha-2C10, alpha-2C2 and alpha-2C4 receptors, respectively). Like other adrenergic receptors, the alpha-2 receptors are activated by endogenous agonists such as epinephrine (adrenaline) and norepinephrine (noradrenaline), and synthetic agonists, which promotes coupling to G-proteins that in turn alter effectors such as enzymes or channels.
The alpha-2 receptors couple to the Gi and Go family of G-proteins. Alpha-2 receptors modulate a number of effector pathways in the cell: inhibition of adenylyl cyclase (decreases cAMP), stimulation of mitogen activated protein (MAP) kinase, stimulation of inositol phosphate accumulation, inhibition of voltage gated calcium channels and opening of potassium channels. (Limbird, L. E. (1988) FASEB J2, 2686–2695, Luttrell, L. M., van Biesen, T., Hawes, B. E., Della Rocca, G. J., and Luttrell, D. K., and Lefkowitz, R. J. (1998) in Catecholamines: Bridging Basic Science with Clinical Medicine (Goldstein, D. S., Eisenhofer, G., and McCarty, R., eds pp. 466–470, Academic Press). The alpha-2 receptors are expressed on many cell-types in multiple organs in the body including those of the central and peripheral nervous systems. They are found in presynaptic or postsynaptic locations with the alpha 2AR being the most extensively expressed of the subtypes.
Alpha-2AAR are the principal presynaptic inhibitory autoreceptors of central and peripheral sympathetic nerves and inhibit neurotransmitter release in the brain and cardiac sympathetic nerves. (Hein, L., Altman, J. D., and Kobilka, B. K. (1999) Nature 402, 181–184). Such inhibition of neurotransmitter release in the brain is the basis for the central hypotensive, sedative, anesthetic-sparing, and analgesic responses of alpha-2AAR agonists (Altman, J. D., Trendelenburg, A. U., MacMillan, L., Bernstein, D., Limbird, L., Starke, K., Kobilka, B. K., and Hein, L. (1999) Mol. Pharmacol. 56, 154–161 and Lakhlani, P. P., MacMillan, L. B., Guo, T. Z., McCool, B. A., Lovinger, D. M., Maze, M., and Limbird, L. E. (1997) Proc Natl Acad Sci USA 94, 9950–9955). Indeed, alpha-2AAR agonists such as clonidine and guanabenz are potent antihypertensive agents which act via central presynaptic alpha-2AAR (Holmes, B., Brogden, R. N., Heel, R. C., Speight, T. M., and Avery, G. S. (1983) Drugs 26, 212–229). The blood pressure and other responses to alpha-2AAR agonists and antagonists, though, are subject to interindividual variation in the human population (Holmes, B., Brogden, R. N., Heel, R. C., Speight, T. M., and Avery, G. S. (1983) Drugs 26, 212–229; Dao, T. T., Kailasam, M. T., Parmer, R. J., Le, H. V., Le Verge, R., Kennedy, B. P., Ziegler, G., Insel, P. A., Wright, F. A., and O'Connor, D. T. (1998) J Hypertens 16, 779–792; Goldstein, D. S., Grossman, E., Listwak, S., and Folio, C. J. (1991) Hypertension 18, III40–III48). Such variation, of course, can be due to genetic variation in the structure of the receptor itself, its cognate G-proteins, the effectors, or downstream intracellular targets.
Of particular interest are physiologic and genetic studies which suggest that altered alpha-2AAR function can predispose individuals to essential hypertension (Holmes, B., Brogden, R. N., Heel, R. C., Speight, T. M., and Avery, G. S. (1983) Drugs 26, 212–229; Dao, T. T.; Kailasam, M. T., Parmer, R. J., Le, H. V., Le Verge, R., Kennedy, B. P., Ziegler, G., Insel, P. A., Wright, F. A., and O'Connor, D. T. (1998) J Hypertens 16, 779–792; Goldstein, D. S., Grossman, E., Listwak, S., and Folio, C. J. (1991) Hypertension 18, III40–III48). Other physiologic functions of the alpha-2AAR are known. For example, the alpha-2AAR act to inhibit insulin secretion by pancreatic beta-cells, contract vascular smooth muscle, inhibit lipolysis in adipocytes, modulate water and electrolyte flux in renal cells, and aggregate platelets (Ruffolo, R. R., Jr., Nichols, A. J., Stadel, J. M., and Hieble, J. P. (1993) Annu Rev Pharmacol Toxicol 32, 243–279). Thus, like what has been shown with beta-AR polymorphisms (Liggett, S. B. (1998) Clin and Exp. Allergy 28, 77–79), potential polymorphisms of the alpha-2AAR may act as risk factors for disease, act to modify a given disease, or alter the therapeutic response to agonists or antagonists.
There has been a considerable research effort to clone and sequence the alpha-2AR. For example, the gene encoding the alpha-2A, alpha-2B, alpha-2C subtypes has been cloned and sequenced. (Kobilka et al. Science 238, 650–656 (1987); Regan et al., Lomasney et al. Proc. Nat. Acad. Sci. 87, 5094–5098 (1994)). These receptors have also been named as alpha-2C10, alpha-2C2 and alpha-2C4, according to their location on chromosomes 10, 4 and 2.
Polymorphisms near the coding regions in the alpha-2A, alpha-2C and dopamine β-hydroxylase (DBH) genes have been reported causing increased levels of norepinephrine in children with attention-deficit hyperactivity disorder (Comings et al. Clin Genet 55, 160–172 (1999)). Indeed, there have been several reports of non-coding region polymorphisms (i.e., in the 5′ and 3′ untranslated region) of the human alpha-2A AR. One report has identified three SNPs in the coding region (Feng et al. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 81, 405–410 (1998). In this work, though, no pharmacologic studies were carried out to determine if these polymorphisms alter receptor function.
Given the importance of the alpha-2AAR in modulating a variety of physiological functions, there is a need in the art for improved methods to identify polymorphisms and to correlate the identity of these polymorphisms with signaling functions of alpha-2AAR. The present invention addresses these needs and more by providing nucleic acid and amino acid polymorphisms, molecules, and methods for identifying the polymorphisms in the alpha-2AAR. The present invention is useful for determining an individual's risk for developing a disease, assist the clinician in diagnosing and prognosing the disease. The present invention also provides methods for selecting appropriate drug treatment based on the identity of such polymorphism.